1. Field of the Invention:
The present invention relates to the preparation of water-soluble, high molecular weight organic polymers/copolymers especially adapted as polymeric flocculants for the treatment of waste and other impure waters, and, more especially, to the preparation of high molecular weight, water-soluble acrylic polymers or copolymers essentially devoid of residual monomer or monomers, and to the products of such preparation.
2. Description of the Prior Art:
It is well known to this art that, for reasons of toxicity, it is exceedingly important to utilize for water treatment, e.g., water purification, organic water-soluble polymers or copolymers containing but very slight amounts of residual monomer or monomers. Indeed, different government regulations have set an acceptable upper limit on the content of residual monomer or monomers (particularly in the case of acrylamide polymers or copolymers) at 0.05% by weight, with respect to the weight of the dry polymer.
The preparation of acrylic polymers or copolymers by photopolymerization under ultraviolet irradiation too is well known to the prior art. Compare U.S. Pat. No. 4,178,221 which features the preparation of water-soluble acrylic polymers of high molecular weight containing little or no free monomer. According to this particular process, an aqueous solution of acrylic monomer or monomers is continuously deposited onto a support in the form of a thin layer, said solution containing a photopolymerization promoter in an amount of 0.005 to 1% by weight with respect to the total weight of the monomer or monomers, and also containing less than 1 mg oxygen per liter of solution; the thin liquid layer is then subjected to irradiation at wavelengths between 300 nm and 450 nm for 1 to 20 minutes, the average power of such radiation being between 20 and 300 watts/m.sup.2, the gaseous atmosphere enveloping the thin liquid layer having an oxygen content of less than 5% by volume and the support being cooled in order to facilitate heat removal. Next, the thin layer thus subjected to the onset of photopolymerization is maintained on the cooled support and under an atmosphere free of oxygen and is then exposed to a second radiation stage, at wavelengths between 300 and 450 nm for 1 to 20 minutes, the average power of such radiation being between 300 and 2000 watts/m.sup.2 ; thence in order to effect removal of the residual monomers present in the resulting, at least partially solidified thin layer, the same is stripped from the support and subjected to yet another stage of irradiation, preferably in a cool air atmosphere, said irradiation being at wavelengths of between 300 and 450 nm for 30 minutes to 3 hours, and with the average power of active radiation being between 20 and 500 watts/m.sup.2. The resultant film is subsequently cut into fragments, dried and ground into powder. This process yields polymer having a content of residual monomer or monomers of less than 0.05%. However, it displays the disadvantage of requiring a lengthy period of third-stage irradiation (30 minutes to 3 hours) under conditions differing from those of the earlier irradiation stages. Thus, such prior art process is both time-consuming, as well as expensive.
It too has been postulated that the aforesaid lengthy period of irradiation of from 30 minutes to 3 hours could be avoided by simply extracting the residual monomer(s) with any suitable solvent, such as dilute methanol. And indeed the ultimate product of photopolymerization may be treated with methanol to dissolve the residual monomers, but this is a lengthy, costly and difficult operation, especially in light of the hazardous and toxic character of methanol.
Also, in published French Application No. 2,453,185 there is described a process for the preparation of acrylic polymers or copolymers having high molecular weights and a very low content of residual monomer or monomers, this being a process of the type according to which a liquid layer of an aqueous solution of an acrylic monomer, or of acrylic monomers, which has a pH of between about 4 and about 14 and which contains a photopolymerization promoter is deposited on a support and the liquid layer is subjected to the action of radiation of wavelengths between about 300 nanometers and about 450 nanometers until a rubbery layer is obtained, the process being characterized in that at least one of the two face surfaces of the rubbery layer obtained is coated with at least one alkali metal sulfite and/or at least one alkali meta metabisulfite, and thence the coated rubbery layer is chopped into flakes, and the flakes are dried and ground.
The powder obtained according to this latter process has a monomer content of less than 0.05%. However, it will be appreciated that a new stage, involving a particular treatment, has had to be introduced into the process.
And in French Pat. No. 2,084,010 a process is described for the preparation of water-soluble acrylic polymers, which process features the intermittent irradiation of a thin layer of acrylic monomers containing various types of photoinitiators. Nonetheless, the patent examples reflect a residual monomer(s) content on the order of 2%.
French Pat. No. 2,194,698 envisages the use of benzil dimethylacetal in the photopolymerization of divers monomers. However, this '698 patent neither suggests nor alludes to the use of benzil dimethylacetal for the polymerization of thin layers of aqueous solutions of unsaturated monomers.